Effects of local dissipation profiles on magnetized accretion disk spectra

TL;DR

This paper models the spectra of high-luminosity black hole accretion disks, showing how different dissipation profiles affect the emitted spectra and can reproduce observed steep power law states.

Contribution

It introduces new dissipation profiles based on shearing box simulations that better match observed X-ray binary spectra, especially the steep power law state.

Findings

Dissipation profiles influence the transition from black body to Comptonized spectra.

Modified dissipation profiles produce power-law spectral tails similar to observations.

Models can replicate features of the steep power law spectral state.

Abstract

We present spectral calculations of non-LTE accretion disk models appropriate for high luminosity stellar mass black hole X-ray binary systems. We first use a dissipation profile based on scaling the results of shearing box simulations of Hirose et al. (2009) to a range of annuli parameters. We simultaneously scale the effective temperature, orbital frequency and surface density with luminosity and radius according to the standard \alpha-model (Shakura & Sunyaev, 1973). This naturally brings increased dissipation to the disk surface layers (around the photospheres) at small radii and high luminosities. We find that the local spectrum transitions directly from a modified black body to a saturated Compton scattering spectrum as we increase the effective temperature and orbital frequency while decreasing midplane surface density. Next, we construct annuli models based on the parameters of a L/L_Edd=0.8 disk orbiting a 6.62 solar mass black hole using two modified dissipation profiles that explicitly put more dissipation per unit mass near the disk surface. The new dissipation profiles are qualitatively similar to the one found by Hirose et al. (2009), but produce strong near power-law spectral tails. Our models also include physically motivated magnetic acceleration support based once again on scaling the Hirose et al. (2009) results. We present three full-disk spectra each based on one of the dissipation prescriptions. Our most aggressive dissipation profile results in a disk spectrum that is in approximate quantitative agreement with certain observations of the steep power law (SPL) spectral states from some black hole X-ray binaries.